Avatar Aang. When first met, he is a scared boy who, from his perspective, just learned he was to be the Savior of the World and must leave home from the only man he knew as a father. Now the world is in danger and so he shirks his responsibilities. Less than a year later, he is a fully competent Avatar, Master of All the Elements. Even without using the Avatar State, he can hold his own against a super-powered Phoenix King Ozai. When he finally activates the Avatar State, it is a no holds contest.
Many people found Katara's sudden leap from being second to Aang in waterbending to being Aang's more skilled trainer rather jarring (the fact that her training was handled entirely offscreen and concluded with the use of As You Know does little to rectify). It helps that they point out that Katara is driven to excel, which enables her to get more out of quality training than her Brilliant, but Lazy beau.
Sokka, who was previously just the Plucky Comic Relief, took his level in badass when he got his space sword, made actual plans for an army and wore that cool wolf armor. It helps that they show that he learns fairly fast given the opportunity and introduce him to lots of teachers. He and his sister didgrow up in an isolated village without access to teachers.
Zuko actually takes several levels in badass during the third season. As the first season went on, he could almost be seen as an Ineffectual Sympathetic Villain. Throughout the second season, he is constantly outmatched by his sister, Azula. During the third season however, he takes a Heel-Face Turn, stands up to his father, Ozai and tells him exactlywhat he thinks about him and that he is leaving to train Aang so that he can defeat him. This hits Ozai's Berserk Button and he fires a lightning bolt at Zuko, which Zuko deflects back at him before leaving. After that, his fights against Azula were a lot more even, enabling him to hold her off long enough to aid in dramatic escapes. And once she began her Villainous Breakdown, she had to present Zuko with a Sadistic Choice in order to even gain an advantage in their Agni-Kai duel. He was actually winning, managing to knock her to the ground, whilst being calm and collected, until she pulled that move.
Zuko's uncle Iroh, already pretty badass, works off the extra weight he'd gained in retirement and takes further levels in badass.
Toph was already a Master Earthbender when the GAang found her. She could take on most any Earthbender, save Bumi, and win easily. How does she become even more badass? By developing Metalbending, something no one in the history of the world had done before.
The first scene of Bumi was back when Aang hadn't vanished from the world. He was a scraggly kid with a weird face and a love of pranks. A century later and he is the strongest Earthbender in the series able to do several amazing feats of bending with his head exposed. He later freed his city in the span of 8 minutes when all the Firebenders were powerless.
Ike goes from "Don't kick the baby!" in season 1 to being part of a plan to steal the Hope Diamond by Barack Obama and John McCain. This is before he runs off to be a pirate having grown bored with middle class life.
Liane Cartman as of "Coon 2: Hindsight." She's actually refusing to be Cartman's personal servant for once, and punishing him for swearing and beating up his friends. It says something that this is a level in badass for her. (She does fall for the LeBron James Technique, however.) As of "HumancentiPad", she has apparently stopped putting up with any of Eric's attitude, making her one of the few (if only) competent parents on the show. Of course, now Eric has started screwing her over the same way he does everyone else.
Kenny as of "Mysterion Rises".
Butters on numerous occasions:
"Super Fun Time": He will not let go of Cartman's hand. For anything.
"Christian Rock Hard": After Cartman's plan to scam the Contemporary Christian community falls flat on his face, breaking up his band and humiliating him in front of a deeply offended audience, Butters (a devoted Christian himself) farts in Cartman's face, then flips him off and says "Fuck you, Eric."
"Imaginationland": Freaking becomes the Messianic Archetype of Imagionationland's by turning the war around with his own, well, imagination, and then completely fucking resurrecting it after a nuclear strike by imagining the whole fucking thing back into existence.
"The Tale of Scrotie McBoogerballs": "The Reason You Suck" Speech he gives the main characters after the novel they wrote makes him a worldwide celebrity.
Taken Up to Eleven with Wendy in "Breast Cancer Show Ever" by standing up to Cartman and then beating the snot out of him at the end.
Played for laughs in the The Fairly OddparentsMade-for-TV Movie, Abra-Catastrophe!, where Cosmo briefly helps Timmy combat an all-powerful Denzel Crocker after watching an 11-second workout video and gaining Hulk Hogan pecs. Played straight in the same movie, with Crocker.
Transformers Generation 1: All the Autobots should qualify for this trope, since they were designed for civilian use. Extensive after-market tuning turned a loser into the Optimus Prime we know and love today.
Wheelie of all people took a level in badass after season 3. He lost the rhyming speech disability, got drunk, and beat up several Decepticons in succession in the Headmasters pilot.
After season 3? Try after The Movie — in the five-part season premiere, he shoots a Sweep right in the face while free-falling, simultaneously breaking from his rhyming to deliver the One-Liner "Galvatron is strong, but Wheelie is mean!"
Galvatron. As Megatron, "Decepticons, REEEETREEEEEEAT!" was practically his catchphrase, uttered about three times per episode. His high-pitched voice (second only to Starscream) didn't add Badass points, either. Then he gets upgraded into Leonard Nimoy temporarily. Then he goes mad, permanently and becomes a much more interesting villain and unpredictable threat. Unfortunately, crazy Galvatron was as likely to pound his own minions as he was the Autobots. Other versions of Megatron get upgraded into Galvatron as well, but for the first Galvatron, it's about the personality change even more than the increase in power.
Keep in mind his final moments as Megatron were actually wiping out a good number of key Autobots, and, after an impressive one on one, mortally wounding Optimus Prime himself.
More recent incarnations of Megatron have themselves become far more intelligent and dangerous in their skills and motives to the point where it feels they don't need an army to win. Perhaps the most notable of them is the Animated Megatron, who is so much of a threat that he considers the Autobots as little more than pests for most of the time (even going so far as to not acknowledge them by name).
Reversing this seemed to be the main point of Transformers Animated, where the Autobots were all workers on a space bridge, and only have a weapon or two each, all of which, with the exception of Prowl's shurikens, had non-combat purposes in mind. This was to keep the Decepticons from suffering from Villain Decay (which is also why there are more human villains). In fact, they're armed to the teeth and it takes all of the Autobots to just take down one or two of them.
Prowl was already pretty badass in Animated with the whole Cyber-Ninja thing (though technically he never finished the training), but he kicked it up a notch in "Five Servos of Doom" when he really seemed to get a handle on the Cyber-Ninja powers. Especially when He destroys a scorebord in a baseball field with only his mind, thus saving himself, Lockdown, and the Starscream clone from being flattened into pancakes.
During Transwarped, Sari of all people took a level in badass. In fact, she had to take a level out of badass, because powering up her newly-discovered-to-be-technoorganic self made her an out of control juggernaut that nearly killed Bumblebee and endangered most of the city. Now she just shoots energy blasts from her hands, can program space bridges, and is actually mature.
Cheetor from Beast Wars got noticeably more capable with his own abilities. Originally just a scout and little skill in combat, he eventually became The Lancer to Optimus Primal during Beast Machines and a competent leader in his own right.
Starscream in Transformers Prime. After spending most of the first half of the second season doing the Humiliation Conga, he discovers the indestructible Apex Armor. He then loses it not many episodes later, but afterwards he gains Red Energon, which he uses to steal the Omega Keys from the Autobots. Then he rejoins the Cons, makes a tactical decision that ultimately denies the Autobots the Omega Lock, and leads a freaking armada to attack the Autobot base, shooting Wheeljack down in the process.
Soundwave would be seen to be this if it wasn't clear in hindsight that he was incredible from the start. For most of the first season he does nothing but act as the silent communications officer for the Decepticons and leaves the ship all of once. Then near the end of the season he absolutely embarrasses Airachnid without moving from his spot when she attempts a coup (after the beatdown he goes back to work). Moving into season 2 he fights Wheeljack to a standstill and displays perfect and instant mastery of Spacebridge technology when he lets Arcee take a flying attack at him and opens a bridge millimeters before him that she flies into. Now in Season 3 he's fought an upgraded Optimus Prime on even air (they were flying) and snagged a 6-inch fossil falling in mid air with another instant portal. Essentially the writers have to be stingy with Soundwave's screentime because he is a veritable force unto himself.
There's also Arcee, in most continuities she is mostly The Chick who sits in the sidelines while everyone else does all the fighting. In Prime she is a straight up Action Girl who gets into more fights as much as everyone, and she's not afraid in taking on bots who are thrice her size and win.
Matt from season 2 of W.I.T.C.H. takes lessons in how to fight from resident Badass Normal Caleb. He's still not great in a fight, but at least he's not completely useless. He takes a further level later in the season when he gains full blown magic powers.
Certain versions like the 2007 CGI movie, have April O'Neil learning their brand of ninjitsu. She's also been a playable character in a few video games. (Probably because some designers felt a need to include a female fighter.)
Serling, a butler robot in Fast Forward, actually saved his master's life in probably one of the most impressive fashions.
In the animated version of Legion Of Superheroes, the entire series takes a level in badass between the first season and the Darker and Edgier second season. Several of the male characters bulk up significantly, especially Superman, Brainiac 5, Timber Wolf, and Lightning Lad (who also Grows the Beard). On top of that, it also introduces a second Future Badass version of Superman. At least in the case of Superman (the original one) this is justified as Brainiac intentionally went back in time to get him, but not as far back in time, so the Superman he brings back is older and more experienced.
Taken even further in season 3 where he fakes his own death for 9 months to investigate a secret project only 4 people are supposed to know exist, and makes the series Big Bad go into a minor rage because he saw his face. He even manages to avert World War 3 and bring peace to the Middle East WITH SNOWCONES.
Here's a fun game: take some buddies who are familiar with the Super Friends and show them either the Justice League episode "The Enemy Below" or "The Terror Beyond". Once they're done, tell them that the crazy blonde badass dude with the hook for a hand is, in fact, the once incredibly lame Aquaman. We would like pictures of their reactions.
Godzilla: The Series. Sure, it's technically his last-surviving offspring. But, the fact that he has nuclear breath and fights other monsters like his Japanese counterpart really helps.
A three-part special has the ORIGINAL American Godzillabeing turned into a surprisingly powerful cyborg, becoming the series' version of Mechagodzilla.
Ron Stoppable from Kim Possible took a severe level in badass the first movie, the Grand Finale and the episodes where he is turned evil.
In the Kim Possible movie, 'A Sitch in Time, the future version of Monique has clearly taken several levels in badass since we last saw her— she's gone from having to duck and weave and run away from every bad guy she faces (when she subs for Ron as Kim's sidekick in the present), to helping Kim kick some serious ass in the future. (Ron is still the one who ultimately saves the day.)
In Batman: The Animated Series, the episode "Mad Love" shows the character Harley Quinn taking a level of badass on multiple people. She utilizes a thrown-away plan by the Joker positively, stabs Batman In the Back (of the neck), and has Batman out to the point where he later admits to Joker, "I have to admit, she came a lot closer [to killing me] than you ever did. Puddin'." Her CMOA vanishes when she gets knocked out a five-story building.
Remember when Cobra Commander of G.I. Joe was a whiny twit of silly schemes? When he destroys Moscow as a show of force, in G.I. Joe: Resolute, you will know you are dealing with a badass Big Bad who would give anyone nightmares.
Valerie Gray started out as a rather spoiled, superficial girl, and by the end of the second season, could kick major ghost butt without her suit using little more than her housekeys.
Arguably Danny himself through Character Development. Compare his first few episodes to the latter ones. In the beginning he was awkwardly stumbling with his powers. By end though, he had enough strength to kick god-like ghost beings!
In Aladdin: The Series, Princess Jasmine was noticeably more of an Action Girl than in the two preceding films. This got even more pronounced when certain spells got put on her:
In "Sandswitch", Jasmine is given memories of a life as a street rat. The new Jasmine is introduced effortlessly beating up four of the royal guards to rescue Abu and Iago.
In "Forget Me Lots", Jasmine's memory is removed and she's told that she is Harmless Villain Abis Mal's daughter, and one of the most evil and feared people in the area. This causes her to instantly become a marvelous Dark Action Girl who takes over the palace almost singlehandedly, then gives herself a Klingon Promotion when she realizes how useless Abis Mal is.
In "Eye of the Beholder", Jasmine is turned into a naga. Even though she's horrified of her snake tail, she puts it to very good use a few times on the way to find a cure.
In "A Sultan Worth His Salt", Jasmine receives warrior training after being kidnapped by a group of Amazons.
This carries over into the final sequel Aladdin and the King of Thieves, where she holds her own against the Forty Thieves, all of which are at least twice her size.
Aelita from Code Lyoko spends the first two seasons as a Damsel in Distress needing the protection of the Hero Secret Service (and sometimes picking the Distress Ball herself, to the point of being considered The Load by part of the fandom). Then by Season 3 she gains an attack power and no longer can be killed just by losing all her Hit Points. Then she gains the abilty to sprout wings and fly. She also progressively gets more confident, turning into a full-fledged Action Girl. She is still XANA's Designated Victim throughout Season 3 and 4 thanks to her MacGuffin Girl status, but now she's certainly fighting back.
Everyone in the opening of season 3, it started with them breaking their friends out of Area 51, hiding out in a cave (With a box of scraps!) take out several high powered military choppers & beat up Juggernaut (Who in season 1 they needed everyone, plus the Brotherhood to take off his helmet so Chuck could mind blast him), Cyclops especially when they go back to school & has his glasses taken, he beats up the jerks who stole them & then later gets stranded in Mexico without his glasses, & survives fighting Wolves! His quote to Logan shows off their newfound badassy "No Logan you trained us, & you know something? We're good, REAL Good."
The Badass level could be rooted back to Season 1's finale, the previous episode had Kurt, Evan & Kitty getting beaten up by Lance, Pietro, & Blob, they come back from this to then not only beat them, but also Destroy a flying villain hideout which itself included a Take a Level in Badass machine.
Henchman 21 from The Venture Bros. is a prime example of this trope. From Season 1 to Season 3, he was blubbery, lazy and inept. But when his best/only friend Henchman 24 was killed, his personality changed radically, and he started training. Now instead of being the Monarch's pudgy, occasional go-to guy, he is officially his most fearsome and competent henchman, now sporting a heavily muscled frame, as well as wrist-blades and greatly improved fighting skill.
His new nickname, "Two-Ton 21", perfectly reflects this. On one hand it sounds like a reference to a monstrous behemoth, but on the other hand it also refers to him still having a bit of a pudgy frame which 21 even groans a bit over.
21's skills have been even further quantified when he takes on Brock Samson and actually puts up a better than decent fight. Brock himself admits as much after beating him. A Crowning Moment Of Awesome for 21 to be sure.
In the episode "Assasinnany 911," Molotov Cocktease attempts to give the Venture boys a lesson in Bad Ass. It doesn't take.
Done in a small way by Sally Impossible after she hooks up with J-J Venture. She goes from a flaky co-dependent (and possibly alcoholic) to a strong-minded young woman who can chew out Richard and co-pilot the Ventron.
Teen Titans' Slade took a level in badass come season 4. After being absent for a season, (Due to being betrayed and apparently killed by his apprentice) he comes back with magical powers and proceeds to beat the Titans senseless without even breaking a sweat.
When he first appears, Willy Watt of Batman Beyond is a typical scrawny nerd. After a few months in juvenile hall, he becomes a buff badboy that even the other kids in juvie are afraid of.
Plankton from Sponge Bob Square Pants by season 7 and onwards. In seasons 1-3 he was a very effective villain, in seasons 4-6 he suffered Villain Decay, then starting with season 7 he returns to his threatening and menacing self, particulary in the episode "Frozen-Face Off", "SpongeBob's Last Stand" and "One Coarse Meal". In "Frozen-Face Off", he organized an entire race as a distraction so he could steal the Krabby Patty formula without trouble and interference and in order to throw off the suspicion, he placed his robot doppelganger to participate in the race and it took all the main characters to stop his plans. In "SpongeBob's Last Stand" he built a super highway which directly led to the Chum Bucket and it drove away all jellyfish from their fields and took over the entire town and almost all of Bikini Bottom's population supported him. It was so evil that Mr. Krabs was going to give the secret formulawillingly. Both episodes were more dramatic and serious than most other episodes in the series. Even in "One Coarse Meal", the episode where Mr. Krabs crossed the Moral Event Horizon by driving Plankton to suicide, Plankton was a very effective villain. He drops a powerful explosive the size of a TV remote on the Krusty Krab's roof; he is said to build a transmutator (a machine with the power to make an element in nature into another element, like iron into gold, fire into air, etc.); and a robotic bomb-shaped object that is actually a disguised, robotic, telescopic claw that can crush a sea creature. And at the end of the episodes, Plankton used a mime to scare Mr. Krabs, who is afraid of mimes, and judging by Krabs' reaction, his phobia of mimes is bigger than Plankton's phobia of whales, since Mr. Krabs was completely irrational, while Plankton managed to stay rational and not go crazy like Krabs did. This unintentionaly foreshadows that Plankton will become much more monstrous and effective and this will lead into the events of the movie, which is the Grand Finale of the series.
SpongeBob himself takes a rather notable level in "Pets or Pests". In the protection of his new pet baby worms, he tackles Larry Lobster to the ground to stop him from eating them and takes a semi-truck that was about to run them over apart.
The episode Spongebob, You're Fired! has the title character saved by a mysterious figure in a Krabby Patty costume known as the Killer Patty, who beats the tar out of competing fry cooks before carrying the little guy off. And who does this masked hero turn out to be? Squidward.
Don't expect the show's haters to admit it, but Ben Tennysondid took some level in badass in Ben 10: Alien Force; in the original show, he was a 10-year old immature kid with a very limited control on his powers, who would easily make himself look ridiculous or ending up in distress when the Omnitrix stopped working. In Alien Force, he has grown up into a responsible leader, has much better control over the Omnitrix, and is a competent fighter even without the Omnitrix, as well as much more skilled at using his aliens. As an annoying side effect however, it resulted in some of his allies and enemies, especially Kevin 11 and Vilgax, to suffer Villain Decay. Presumably because of fans' reaction, Ben then Took a Level in Dumbass in season 3 due to a fail attempt to make him more like his original series' self.
Also, some of the B-level members of the Rogues Gallery have gotten badder. Dr. Animo temporarily rules a world. Also, Vilgax absorbs the powers of the champions of worlds he conquers, and returns a lot tougher.
Scooby-Doo is no stranger to this trope, but Scooby-Doo! Legend of the Phantosaur comes to mind. Shaggy, acting under hypnosis, beats the crap out of an entire biker gang, wins a motorcycle race after rescuing his opponent, and navigates the gang (plus a few more) through a pit full of snakes during a mine explosion.
And at the end, he ends up escaping without the hypnosis, and saves the rest of the gang.
Bloom, in the season one finale of Winx Club, takes down Icy, an older, more experienced spellcaster who at this point really just wants to kill Bloom once and for all, by manifesting the abilty to teleport, and just popping up around Icy until she burns out her magic trying to knock her out of the air. Considering that Bloom hadn't even been at Alfea for a year (and wasn't terribly great at magic up to this point), this definitely qualifies.
Prince Desmond, in the "Make Way for Miss Nettle" episode of Sofia the First. He starts out as a coward who fears almost everything, including wet socks, but ends up tackling a powerful fairy to take her wand.
The Vegimals in "The Very Vegimal Christmas". Building a new GUP? Check. Saving the Octonauts from danger? Check. Cooking the Christmas dinner? Check. Looking cool while doing so? CHECK!
"Adventure Time". Cinnamon Bun, the once bumbling and naive servant of Princess Bubblegum, earned himself a level. His time with Flame Princess made him into the new champion of Finn's former flame. He one ups Finn, fights of the mooks of the Fire King, taught his pet fire wolf to fly, and possibly started a relationship with Flame Princess.
Amberley of The Dreamstone. Similar to Rufus, Amberley was thoroughly useless in the second season, and even in the first had minimal moments of effectiveness, usually playing a Distressed Damsel. In the third and forth season, Amberley becomes more tactical, often forming team strategies, with her and Rufus being the most consistent reason the Urpneys are stopped.
While not as consistent, the Urpneys also had a few more evident cases of being Not So Harmless in the later seasons (they are thoroughly unstoppable in "The Spidermobile" for example). Combined with Zordrak's more ambitious motives for the capturing the title stone (from giving the Noops bad dreams to taking over the universe), there was a fair bit more emphasis on the heroes being in legitimate danger.